Yuuki Tetsuya was the captain of the Seidou High School Baseball Team. He grew up in Tokyo Metropolis and graduated from Akadou Junior High. Observing the team's discipline, Yuuki decided to enroll at Seidou. During his Junior high however, he was not given much opportunity because of his small build despite his strong fighting spirit.

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Yuuki is the most composed core member of the team who greatly supported the hearts of his teammates with his straightforward personality and calm words and actions. He is the type to rest his case through performance. Occasionally, Yuuki startles his teammates when he gets excited or irritated when people make remarks about the team. He is dedicated to enhancing his batting and fielding for the furtherance of the team and doesn't hesitate to lend his help even after their retirement.

He and Jun are the senior members with high senses of responsibility. Right after Tanba's injury, they were the first people challenged by Kataoka to renew the team's motivation. Their response rightly brought back the team's spirit.

As the team's captain, he leads them through his performance. He knows his role as the fourth batter and does his best to meet everyone's expectations of a captain. Yuuki has very strong presence especially to Mei Narumiya. He is the only member of Seidou Narumiya seriously faced at their first encounter during the finals.

Yuuki was not an outstanding player from the start. Although he claimed to have experience and expertise (out of confidence) in fielding, Yuuki struggled in his first year together with Isashiki and the others.

In order to cope with his weakness, he trained himself vigorously in and out of the field. He swings at least 500 a day in order to strengthen his batting skills, inspiring his batchmates to train as much as he did.

His training paid off as he was the first in his batch to produce good results in their first game by getting a home run. His steady performance and skills impressed opponents and team mates so much that when they retired, they unanimously voted him to become the next captain.

He later attend Meijin University and is a 1st year player of the University's baseball team.[6] He participates a practice match against the Tokyo Senbatsu team where he faced his high school friends and foes such as Miyuki and Mei.

Yuuki is his first rival upon entering Seidou baseball team yet he was his first real buddy to inspire him to practice even more, especially in batting. When Yuuki became the captain, he was appointed as vice- captain together with Masuko. Occasionally, Isashiki is the first one to notice Yuuki's sudden change of moods (resulting from his intense aura).

Miyuki is Yuuki's Shogi buddy. Even though he always loses, he never gives up playing (which is attributable to his determination). He is one of the seniors who hangs out in Miyuki's room. On field, these two rarely have casual communication which shows each others trust with regards to their role.

During Yuuki's first year, he's called one of the greenhorns. As time passes by, Coach Kataoka noticed their determination on and off the field and became stricter than usual in training. Yuuki's hard work paid off after a great homerun during a practice game (their second year) and Kataoka was impressed. He then supported the seniors' unanimous decision appointing Yuuki as captain. To quote: "I don't need you to handle everything in the huge team perfectly, you just need to lead everyone with your performance." [7] Coach Kataoka gives big credit to Yuuki's performance on the field. Occasionally, he tells the boys to observe Yuuki and learn his responses to difficult offensive situations. Yuuki has very high respect for the coach and decided to take a similar path to him. He rejected the opportunity to go pro and decided to attend the university Kataoka graduated from so he can come back to Seidou later as a coach.

Yuuki and Harada converse pleasantly during the Seido vs Yakushi match; talking about the current situation of their respective teams. Yuuki recommended Miyuki as captain due to the fact that Seido lost to Inashiro whilst Harada was captain thus Yuuki wanted Miyuki to be a captain that can surpass the level of Harada. They both seem equally shocked to finding out that Miyuki was injured during the match

As someone unanimously voted by his seniors and called an "unshakable pillar" by his team mates, Yuuki is the most reliable member of Seidou High Baseball team. He has outstanding presence just by standing in the batter's box. Other schools are very cautious against him, as he is able to hit any kind of pitch and as a result, is often intentionally walked by his opponents.

Batting: As the fourth batter, he is aware that hitting cleanup requires an exceptional level of talent, and the ability to deliver big hits in important situations (such as the bases loaded with two out). He is able to successfully cope with every course or every type of pitch, on top of being a slugger - that is what’s called thoroughness without flaw. His mental strength in critical situations really stands out, and the team scored a lot of runs when he's at bat.[8]

Yuuki, among all team members "exudes the right aura" when it comes to baseball. One unique training he does is "image batting," and all that is left is hitting real balls during the game.

He is the only batter who hit Narumiya's pitch during the Seidou vs Inashiro game when he was still a second year. Narumiya is well aware that he is the team's key player who needs to be shut down when at bat. During the Summer tournament, the two had another face-off wherein Yuuki was struck out twice before hitting a bases loaded-2-run RBI giving Seidou temporary relief during the offense.

Narumiya Mei aside, Yuuki was never struck out without a fight. He is regarded as the team's best hitter, thus, had been the steady clean-up hitter. During games, Yuuki has the most reliable offensive performance who hits with instinct and experience.